During development, neurons establish synaptic connections in a precise and specific manner. In many systems, the correct connections are made because growing axons have the ability to choose correctly among several available pathways and thus arrive at the correct target sites. The long-range goal of this research is to further our understanding of specificity and axonal guidance by elucidating how the axons of sensory neurons choose among pathways in the chick hindlimb during development. The experiments proposed here will employ a tissue culture system to examine mechanisms at a more cellular level than has been possible in the intact embryo. The neurons to be studied will be labeled prior to culturing with fluorescent carbocyanine dyes, so that it is possible to unambiguously identify the different types of neurons after they are placed into culture. The three major aims are: 1). To elucidate how sensory neuron axons may use other axons as cues, and more specifically, how motoneurons axons may guide sensory neuron axons along pathways in the limb, as in vivo studies suggest they do, time-lapse videotaping will be used to examine the behavior of sensory neuron growth cones as they encounter the processes of motoneurons. 2). To elucidate why motoneuron axons never grow to the skin whereas sensory neurons axons normally grow to skin and to muscle, a series of experiments will examine if motoneuron and sensory neuron axons are attracted by the appropriate type of target tissue. 3) To determine whether muscle sensory neurons and cutaneous sensory neurons are intrinsically specified, a series of experiments will assess if the two types of sensory neurons differ either in terms of the responses of their growth cones to motoneuron axons or to other sensory neuron axons or with respect to their growth to the two different types of target tissues. If differences are found, sensory neurons from younger embryos will be examined to ascertain if the observed differences are intrinsic or if they are a consequence of the neurons having become specified by contact with their peripheral target tissues. Together these studies will increase our understanding of the differences between different types of neurons and of the cellular interactions that may play a role in pathway selection during development. Understanding these processes is important because abnormalities in these processes during development would result in defects in the organization, and consequently the function, of the nervous system.